Fabry-perot opitcal switch having a saturable absorber

ABSTRACT

A Fabry-Perot optical switch includes a saturable absorber surrounded by a pair of mirrors. Coupled to the saturable absorber is an input waveguide, an output waveguide, and a control beam waveguide. In the absence of light input to the control beam waveguide, the saturable absorber prevents an input signal on the input waveguide from passing through and being output on the output waveguide, thus placing the switch in an “off” state. In the presence of light input to the control beam waveguide and incident on the saturable absorber, the saturable absorber allows the input signal to pass through and be output on the output waveguide, or be reflected and output on the output waveguide, thus placing the switch in an “on” state.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/737,470, filed on Dec. 18, 2000 and entitled “Optical SwitchHaving a Saturable Absorber”.

FIELD OF THE INVENTION

The present invention is directed to optical communications. Moreparticularly, the present invention is directed to an optical switch foran optical network.

BACKGROUND INFORMATION

The enormous increase in data traffic, largely due to the growth inInternet traffic, has spurred rapid growth in broadband communicationtechnologies. Fiber optics, which offers the largest bandwidth of anycommunication system, is the medium of choice for carrying the multitudeof data now being sent through networks. While fiber can theoreticallycarry over 50 terabits per second, current optical communication systemsare limited to 10 gigabits per second due to the limitations of theswitching nodes.

Switching nodes consist of systems dedicated to switching the opticalsignals between lines as well as providing other signal processingfunctions, such as amplification and signal regeneration. Switchingnodes include components such as optical switches, add/dropmultiplexers, channel converters, routers, etc.

Prior art “optical” switches used in switching nodes are typically notentirely optical and therefore operate relatively slowly and havelimited bandwidth. One type of known prior art switch is anopto-mechanical switch. Opto-mechanical switches use moving (e.g.,rotating or alternating) mirrors, prisms, holographic gratings, or otherdevices to deflect light beams. The mechanical action may involvemotors, or piezoelectric elements may be used for fast mechanicalaction. For example, Lucent Corporation and other companies haveintroduced a type of opto-mechanical switch referred to as amicro-electro-mechanical switch (“MEMS”). MEMS consist of arrays ofactuated micro-mirrors etched onto a silicon chip in a similar manner tothat of electrical integrated circuits. The mirrors change angle basedupon an electrical signal and route an incident optical signal to one ofmany output fibers.

Another example of an opto-mechanical switch is a device from AgilentTechnologies that steers optical signals through the controlledformation of gas bubbles within a liquid waveguide. A bubble is formedat the junction of one input and several output waveguides. The bubblewill reflect an optical signal down one output while a lack of bubblewill allow the signal to propagate through another waveguide.

A major limitation of opto-mechanical switches is low switching speeds.Typical switching times are in the millisecond range. The advantages ofopto-mechanical switches are low insertion loss and low cross-talk.

Other prior art devices use electro-optic materials which alter theirrefractive indices in the presence of an electric field. They may beused as electrically controlled phase modulators or phase retarders.When placed in one arm of an interferometer, such as a Mach-Zenderinterferometer, or between two crossed polarizers, the electro-opticcell serves as an electrically controlled light modulator or a 1×1(on-off) switch. The most prevalent technology for electro-opticswitching is integrated optics since it is difficult to make largearrays of switches using bulk crystals. Integrated-optic waveguides arefabricated using electro-optic dielectric substrates, such as LithiumNiobate (“LiNBO₃”), with strips of slightly higher refractive index atthe locations of the waveguides, created by diffusing titanium into thesubstrate. The major drawbacks of Lithium Niobate technology is the highexpense of the material and difficulty in creating low loss waveguideswithin it.

Liquid crystals provide another technology that can be used to makeelectrically controlled optical switches. A large array of electrodesplaced on a single liquid-crystal panel serves as a spatial lightmodulator or a set of 1×1 switches. The main limitation is therelatively low switching speed.

Other prior art optical devices include acousto-optic switches which usethe property of Bragg deflection of light by sound. An acoustic wavepropagating along a dielectric surface alternatively puts the materialin compression and tension. Thus, the acoustic pressure waveperiodically alters the refractive index. The change in the refractiveindex is determined by the power of the acoustic wave, while the periodof the refractive index change is a function of the frequency of theacoustic wave. Light coupled with the periodically alternatingrefractive index is deflected. A switching device can be constructedwhere the acoustic wave controls whether or not the light beam isdeflected into an output waveguide.

Some prior art optical devices use magneto-optic materials that altertheir optical properties under the influence of a magnetic field.Materials exhibiting the Faraday effect, for example, act aspolarization rotators in the presence of a magnetic flux density B. Therotary power ρ (angle per unit length) is proportional to the componentB in the direction of propagation. When the material is placed betweentwo crossed polarizers, the optical power transmission T=sin²Θ isdependent on the polarization rotation angle Θ=ρd where d is thethickness of the cell. The device is used as a 1×1 switch controlled bythe magnetic field.

Finally, prior art optical devices do exist that can be considered“all-optical” or “optic-optic” switches. In an all-optical switch, lightcontrols light with the help of a non-linear optical material. Theyoperate using non-linear optical properties of certain materials whenexposed to high intensity light beams (i.e., a slight change in indexunder high intensities).

FIG. 1 illustrates a prior art all-optical switch 20 that uses aninterferometer. Switch 20 includes material 14 that exhibits the opticalKerr effect (the variation of the refractive index with the appliedlight intensity) which is placed in one leg of a Mach-Zenderinterferometer. An input signal 10 is controlled by a control light 16.As control light 16 is turned on and off, transmittance switch 20 atoutput 12 is switched between “1” and “0” because the optical phasemodulation in Kerr medium 14 is converted into intensity modulation.

FIG. 2 illustrates a prior art all-optical switch 30 that uses anoptical loop. Switch 30 is a non-linear optical loop mirror (“NOLM”)that includes a fused fiber coupler (splitter) 34 with two of its armsconnected to an unbroken loop of fiber 32. A signal arriving at theinput 36 to coupler 34 is split and sent both ways around fiber loop 32.One of the lengths of loop 32 contains a Kerr medium. The Kerr medium ispumped via another high intensity control beam that alters therefractive index of the material and thus slightly changes the speed atwhich the signal beam propagates through. When the two signal beamsrecombine at the other end of loop 32 interference effects determine theamplitude of the output 38. Although a NOLM operates at high speeds(tens of picoseconds), it requires long lengths of fibers and is notreadily integratable.

The retardation between two polarizations in an anisotropic non-linearmedium has also been used for switching by placing the material betweentwo crossed polarizers. FIG. 3 illustrates a prior art all-opticalswitch 30 using an anisotropic optical fiber 42 that exhibits theoptical Kerr effect. In the presence of a control light 43, fiber 42introduces a phase retardation π, so that the polarization of thelinearly polarized input light 45 rotates 90° and is transmitted atoutput 46 by an output polarizer 48. In the presence of control light43, fiber 42 introduces no retardation and polarizer 48 blocks inputlight 45. A filter 44 is used to transmit input light 45 and blockcontrol light 43, which has a different wavelength.

FIG. 4 illustrates another prior art all-optical device 50 that usesliquid-crystal. Device 50 includes an array of switches as part of anoptically addressed liquid-crystal spatial light modulator 52. A controllight 54 alters the electric field applied to the liquid-crystal layerand therefore alters its reflectance. Different points on theliquid-crystal surface have different reflectances and act asindependent switches controlled by input light beams 58 and output asoutput light beams 56. Device 50 can accommodate a large number ofswitches, but is relatively slow.

Still another all-optical switch is based on an optically pumpedSemiconductor Optical Amplifier (“SOA”). A SOA is a laser gain mediumwithout a resonator cavity. A SOA-based switch operates similar to theNOLM in that it operates on interference between laser beams. Like theNOLM, SOA-based devices have significant loss and require high operatingpower. They also suffer other non-linear effects including frequencyaddition, which has the effect of switching the data to a differentwavelength channel. These detriments have prevented SOAs from beingcommercially viable.

Based on the foregoing, there is a need for an all-optical switch havinglow power requirements and fast switching speeds.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a Fabry-Perot optical switchthat includes a saturable absorber surrounded by a pair of mirrors.Coupled to the saturable absorber is an input waveguide, an outputwaveguide, and a control beam waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art all-optical switch that uses aninterferometer.

FIG. 2 illustrates a prior art all-optical switch that uses an opticalloop.

FIG. 3 illustrates a prior art all-optical switch using an anisotropicoptical fiber that exhibits the optical Kerr effect.

FIG. 4 illustrates a prior art all-optical device that usesliquid-crystal.

FIG. 5 is a perspective view of an optical switch in accordance with oneembodiment of the present invention.

FIG. 6 is a graph illustrating the normalized power vs. the absorptioncoefficient of one embodiment of a saturable absorber material.

FIG. 7 is a graph illustrating the relationship between front mirrorreflectivity (“M1”) and rear mirror reflectively (“M2”) and absorptioncoefficient of a cavity absorber vs. the absorption of a switchingdevice in accordance with one embodiment of the present invention.

FIG. 8 is a sectional view of a reflective based Fabry-Perot opticalswitch in accordance with one embodiment of the present invention.

FIG. 9 is a perspective view of a Fabry-Perot switch that is fabricatedon a substrate.

FIG. 10 is a perspective view of a Fabry-Perot switch in accordance withanother embodiment of the present invention.

FIG. 11 is a perspective view of a Fabry-Perot switch in accordance withanother embodiment of the present invention.

FIG. 12 is a perspective view of a counter-propagating Fabry-Perotswitch that includes a circulator and an input/output waveguide inaccordance with another embodiment of the present invention.

FIG. 13 is a sectional view of a switch in accordance with anotherembodiment of the present invention.

FIG. 14 is a sectional view of a switch in accordance with anotherembodiment of the present invention.

FIG. 15 is a graph illustrating the effect of multiple cavities on thedevice reflectivity.

FIG. 16 is sectional view of a normally “off” transmission Fabry-Perotswitch in accordance with one embodiment of the present invention.

FIG. 17 is a sectional view of a Fabry-Perot switch in accordance withanother embodiment of the present invention.

DETAILED DESCRIPTION

One embodiment of the present invention is an optical switch thatincludes a saturable absorber that functions as an active region. Theswitch has an optical input and output, and is controlled by an opticalcontrol signal.

FIG. 5 is a perspective view of an optical switch 60 in accordance withone embodiment of the present invention. Switch 60 includes a slab ofsaturable absorber material (“SA material”) 67 formed on a substrate 62.Coupled to SA material 67, and also formed on substrate 62, is an inputwaveguide 68, a control beam waveguide 64 and an output waveguide 63. Inoperation, an optical input signal 69 is input to input waveguide 68. Inan “off” state no control signal is input to control waveguide 64, andtherefore no control beam is incident upon SA material 67. In thisstate, SA material 67 is highly absorbing and input signal 69 is notoutput on output waveguide 63. In an “on” state a control beam 66 isinput to control waveguide 64. Control beam 66 is a high intensity beamwhich when incident on SA material 67, causes SA material 67 to becomemore transparent. This allows input signal 69 to be output on outputwaveguide 63 as an output signal 65.

In general, a saturable absorber such as SA material 67 is a materialthat displays a reduction in the absorption coefficient at theoperational wavelength with increased incident light intensity. Thebehavior of such a material can be modeled as a two state system, i.e.,a system possessing two quantum states of different energies that anelectron can exist in. In the natural state of the material, one inwhich no light is incident upon the material, all electrons lie in thelower energy state. An incident photon having a wavelength (henceenergy) that corresponds to the energy difference between the quantumstates will be absorbed if it excites an electron from the lower energylevel to the upper energy level.

An electron in the upper state will drop back to the lower energy levelin one of two ways. It can (1) spontaneously drop back and releaseenergy as heat (referred to as “nonradiative recombination”) or as aphoton of the same wavelength that originally excited it (referred to as“spontaneous radiative recombination” or “spontaneous emission”) or (2)interact with another photon, having the wavelength corresponding to theenergy difference between quantum states, that forces the electron downto the lower energy level by the release of two photons (referred to as“spontaneous emission”). The average time the electron remains in theupper level (assuming the drop from the upper state to the lower stateis by spontaneous recombination) is given by the relaxation constant(τ).

At low light intensities there is a much higher probability of anelectron being excited to an upper energy level than an electron beingforced down to the lower energy level because at low light intensitiesvery few electrons exist in the upper state. At higher light intensitiesa higher fraction of the electrons build up in the upper state. Becausemore electrons exist in the upper state there is a larger probability ofan electron being forced to a lower energy level. At the limit(extremely high light intensities) an equal number of electrons exist inthe upper state as in the lower state. At this point there is an equalprobability of an electron in the lower energy levels jumping to theupper energy level (absorbing a photon) as an electron in the upperenergy level interacting with a photon and dropping to the lower energylevel releasing two photons. If both processes are considered there isno net reduction of the number of photons. Hence, the absorption fallsto zero.

A saturable absorber such as SA material 67 differs from, for example, anon-linear material. As discussed, a saturable absorber involves thetransitions of electrons between quantum states. In contrast, non-linearmaterials, instead of relying on transitions, involve the non-linearreaction due to the electric field of the photons at high photon fluxes(i.e., high light intensity). This reaction is called the electricpolarization (P). Because a saturable absorber requires a transitionbetween states, it is highly selective as to which wavelength it canoperate in (i.e., only wavelengths that correspond to an electronictransition can saturate a saturable absorber).

In one embodiment, input signal 69 carries information and is generallya relatively less intense beam. Control beam 66 is generally arelatively more intense beam that has enough power to alter theabsorption of SA material 67, thus allowing input signal 69 to eithertransmit or not transmit through the output of switch 60.

In one embodiment of the present invention, both input signal 69 andcontrol beam 66 are at the same wavelength and have the samepolarizations. However, the two beams must be distinguished bypropagation direction (i.e., both the signal and control beams cannotboth end up propagating down the same output). Therefore in thisembodiment, input signal 69 and control beam 66 intersects at SAmaterial 67 in a perpendicular direction or in a counter-propagatingdirection.

SA material 67 has a relatively high absorption in the “off” state and arelatively low absorption in the “on” state. For example, in oneembodiment, SA material 67 has an isolation of approximately 20 dB(i.e., the power transmitted in the “on” state is 100 times greater thanin the “off” state) and an insertion loss of less than approximately 1dB (i.e., the optical signal loses less than 20% of its power as ittravels through switch 60). In this embodiment, switch 60 should bebetween 80% and 90% transmitting in the “on” state and between 0.08% and0.09% transmitting in the “off” state, because the power required toattain transparencies higher than 90% increases dramatically.

The applicable equation for determining the absorption requirements forSA material 67 is as follows:

I_(out) =I _(in) e ^(−αd)

Where α is the absorption coefficient and d is the length of SA material67. In an embodiment where SA material 67 has a length of 10 microns, anabsorption coefficient of approximately 4700 cm⁻¹ is required for the“off” state (0.9% transmitting) and 100 cm⁻¹ for the “on” state (90%transmitting). FIG. 6 is a graph illustrating the normalized power vs.the absorption coefficient of one embodiment of SA material 67.

The intensity of control beam 66 required for the change from an “on” to“off” state is dependent upon the saturable absorber properties of SAmaterial 67, especially the optical cross section of SA material 67.Typical intensities of control beam 66 in one embodiment range from ashigh as 90×10⁶ W/cm² to as low as 100×10³ W/cm². The actual powerrequired by switch 60 is then determined from the intensity times thecross sectional area of SA material 67. A SA material with a crosssectional area of 1 square micron would require between 900 milliwattand 1 milliwatt of power.

The speed of switch 60 is limited by the rate at which SA material 67can reach transparency upon illumination and then decay back to itsabsorbing state when control beam 66 is turned off. In some embodiments,this rate can range from approximately 100 femtoseconds to 100picoseconds.

One embodiment of SA material 67 is a composite material containingsemiconductor nanocrystals (referred to as “quantum dots”) contained ina glass or silicon matrix. Quantum dots interspersed within a matrixmaterial offer an opportunity for an ideal saturable absorber formultiple reasons. For one, the quantum states of the quantum dots can beengineered to correspond to any wavelength simply by altering theirsize. Further, the density of quantum states (i.e., the number ofelectrons per unit volume that are able to jump from one quantum stateto another) are much lower than in bulk semiconductor materials.Therefore, a lower intensity incident light beam is required for it tosaturate. Further, quantum dots eliminate slower excitations that occurat high light intensities such as a two photon absorption that exists inbulk semiconductors. Therefore, the use of quantum dots enables a fast,low power (low intensity), and tunable saturable absorber.

In one embodiment, the quantum dots are comprised of Lead Sulfide (PbS)or Lead Selenide and are approximately 5 nanometers in diameter. In afurther embodiment, the quantum dots are 5.7 nanometers in diameter.This size of the dots results in a large change of absorption withintensity while maintaining fast switching speed. The intensity of lightrequired to saturate SA material 67 depends on the size and compositionof the dots, as characterized by the optical cross section of SAmaterial 67. The concentration of dots determines how thick a slab ofmaterial (quantum dots in glass) is required to produce a given changein intensity of the signal. In one embodiment, a thickness of 0.1 cm isrequired to arrive at a 20 dB signal change (assuming 50% saturation).Increasing the dot density allows the same change with a thinner device.The absorption length (α₀ ⁻¹) is related to the optical cross section(σ₀)and the number density (dots per volume) of dots N_(d) by:

 α₀ =N _(d)σ₀

A limitation exists to the concentration of dots within the matrixmaterial because it is not possible to pack dots any closer than whenthey are touching. The densest packing configuration is theface-centered cubic (“FCC”) lattice which has a packing density of 0.7.

In one embodiment, the quantum dots are produced in a glass matrix. Theglass matrix material is beneficial for two reasons: (1) it istransparent to the light which is to be absorbed by the dots thusallowing the signal to be transmitted when switch 60 is in the “on”mode; and (2) the glass, having a much larger band gap than the quantumdot material, acts to confine the electron-hole pairs. This quantumconfinement allows the requisite absorption spectrum to be obtained. Inother embodiments, the matrix material is a plastic, or a semiconductorthat is transparent to the operational wavelengths. Other possiblematrix materials include Silicate, Borosilicate, and Phosphosilicateglasses, Polymethyl methacrylate (PMMA), Acrylic, polyamine polymers,and semiconductors including Silicon, Silicon Carbide, Cadmium Sulphide,Cadmiun Selenide, Cadmium Telluride, Zinc Sulphide, Aluminum Arsenide,Aluminum Phosphide, Gallium Arsenide.

In one embodiment, cladding is added to the quantum dots. The purpose ofthe cladding is to greatly increase the optical cross-section of thecore semiconductor quantum dot, thus decreasing the optical powerrequired for saturation as well as decreasing the relaxation time. Anelectrically conducting cladding material (like a metal) locallyincreases the light intensity within the core semiconductor, thusenhancing the absorption cross section. A semiconductor claddingmaterial acts as a surface passivating agent and reduces the number oftrapped states, which increases the absorption cross section.

The band-gap energy of the cladding material is wider than the band-gapof the core semiconductor. In one embodiment, switch 60 has anoperational wavelength of 1500 nm (0.827 eV). In this embodiment,suitable semiconductor cladding materials include Silicon (Si), SiliconCarbide (SiC), Cadmium Sulfide (CdS), Cadmium Selenide (CdSe), ZincSulfide (ZnS), Zinc Selenide (ZnSe), Zinc Telluride (ZnTe), AlAs, AIP,AlSb, GaAs and InP. In addition, other materials that include metalssuch as Ag, Au and Al are appropriate for use as cladding materials.

The thickness of the cladding coating determines the enhancement of theabsorption coefficient of the quantum dot material. The parameterdescribing the coating thickness is the ratio of the core radius to theshell radius (“arat”). Typical values of arat are between 0.7 and 0.85.Thus for core radii between 2.5 nm and 5.0 nm (appropriate for PbS), ashell thickness between 0.5 nm and 2.5 nm gives the desired enhancement.In one embodiment, the quantum dots are manufactured using a thermalprecipitation process that involves dissolving some amount ofsemiconductor material in a molten glass. The melt is controllablycooled until the quantum dots begin to precipitate out in the form ofnano-crystals. A method for manufacturing quantum dots using a thermalprecipitation process is disclosed in, for example, P. T. Guerreiro etal., “PbS Quantum-Dot Doped Glasses as Saturable Absorbers for ModeLocking of a Cr:Forsterite Laser”, Appl. Phys. Lett. 71 (12), Sep. 22,1997 at 1595.

In another embodiment, SA material 67 is manufactured using a colloidalgrowth process that involves growing nano-crystal quantum dots in asolution. Specifically, semiconductor precursors are introduced into aheated surfactant solution. The precursors crack in the solution and thesemiconductors combine to form the nanocrystals. The quantum dots canthen be removed from the solution and combined with a powdered glasssolution. The powdered glass, referred to as a “sol-gel” can be shapedinto a variety of forms. The sol-gel can be sintered into a large block,drawn and sintered into a fiber, or spun on a substrate and sintered toform a thin film. A method for manufacturing quantum dots using acolloidal growth process is disclosed in, for example: (1) U.S. Pat. No.5,505,928, entitled “Preparation of III-V Semiconductor Nanocrystals”;(2) Nozik et al., “Colloidal Quantum Dots of III-V Semiconductors”, MRSBulletin, February 1998 at 24; and (3) Hao et al., “Synthesis andOptical Properties of CdSe and CdSe/CdS Nanoparticles”, Chem. Mater.1999, 11 at 3096.

In one embodiment, substrate 62 of switch 60 is made of semiconductor orglass and is less than 1 mm thick. Waveguides 63, 64 and 68 of switch 60in accordance with one embodiment are in the form of optical dielectricwaveguides consisting of transparent glass, polymer, or semiconductormaterials transparent to the wavelength at which switch 60 is operatingat (i.e., the semiconductor band-gap is greater than the energy of theoperation wavelength photon energies).

The waveguides may be in the form of integrated ridge or buried typewaveguides or integrated waveguides based upon photonic crystals. In theridge and buried waveguides embodiments, the guiding conditions dictatethat the waveguide material have a higher dielectric constant (i.e.,higher index of refraction n) than the cladding material index. In otherembodiments, the waveguide may be of the form of an optical fiber. Inone embodiment, the cross sectional areas of the waveguide are of thesize to support a guided wave at the operational wavelength. In variousembodiments, the dimensions of the waveguides are between 0.5 micron and10 microns in diameter depending on the waveguide material, operationalwavelength, and number of modes that are transmitted through thewaveguide. The input waveguide should have the same cross sectionaldimensions as the output waveguide.

Specific waveguide materials include but are not restricted to Silicate,Borosilicate, and Phosphosilicate glasses, Polymethyl methacrylate(PMMA), Acrylic, polyamine polymers, and semiconductors includingSilicon, Silicon Carbide, Cadmiun Sulphide, Cadmiun Selenide, CadmiumTelluride, Zinc Sulphide, Aluminum Arsenide, Aluminum Phosphide, GalliumArsenide. In most embodiments, the probable semiconductor to be used asa waveguide is silicon or gallium arsenide. The reason that the abovematerials are transparent is that the bandgap energy is greater thanthat of the energy of the incident photons.

In another embodiment of the present invention, an optical switchincludes a Fabry-Perot resonator in which a resonator cavity is formedby placing the saturable absorber material within a mirrored cavity. Thedimensions of the cavity are such that it is resonant with theoperational wavelength (i.e., 1500 nm). The cavity-based switch can beused in a transparent mode in which the optical transmission is alteredfrom a low value (between 0.08% and 0.09%) in the “off” state to ahigher value (between 80% and 90%) in the “on” state. The switch canalso be configured in a reflective mode in which the reflectivity of thedevice is at a low value in the “off” state (between 0.08% and 0.09%reflective) and is a high value in the “on” state (between 80% and 90%reflective). These values are chosen because of standard requirementsthat the insertion loss (i.e., the power loss of the inputted datasignal in the “on” state) be less than 1 dB (greater than approximately80%). In addition, the isolation (i.e., the difference between the “on”and “off” states) should be at least 20 dB (the “on” state has 100 timesthe output power of the “off” state).

In the Fabry-Perot embodiments, the saturable absorber material, whereabsorption occurs, is placed between two parallel mirror structuresforming the Fabry-Perot cavity for the data containing optical signal.Another microcavity may be formed for the control optical pulses toenhance the absorption of the control beam.

The use of the Fabry-Perot cavity device architecture greatly enhancesthe non-linear absorption effects of the saturable absorber material. Ineffect, less of a change in the absorption coefficient is required forthe same levels of the “on” and “off” states. Control over theperformance characteristics of the device are governed by thereflectivity of both the front and rear mirrors as well as theabsorption of the saturable absorbing material in the “off” state (i.e.,when there is no control light incident upon the material).

The reflectivity of the mirrors in the Fabry-Perot switch is designed toenable the maximum change in either reflection or transmission,depending on the device design. FIG. 7 is a graph illustrating therelationship between front mirror reflectivity (“M1”) and rear mirrorreflectivity (“M2”) and absorption coefficient of the cavity absorbervs. the absorption of the switching device. This shows how thereflectivity of a reflection based Fabry-Perot device changes as theabsorption coefficient of the saturable absorber is reduced uponillumination by the control beam.

The Fabry-Perot devices can be configured in either a normally “off”mode, in which the device either transmits or reflects the signaloptical pulse only when the control optical pulse is incident upon thedevice, or in the normally “on” mode, in which the optical signal pulseis transmitted or reflected only when there is no control optical signalincident upon the device. The latter configuration performs the logicalinvert function.

Normally “Off” Reflectivity Based Configuration

The normally “off” reflective based Fabry-Perot device operates byabsorbing the optical data pulse when there is no control optical signalsimultaneously incident upon the active region (the “off” state). Whenthere is a control beam simultaneously incident upon the active region(the “on” state) the absorption coefficient of the active material isgreatly reduced and the data signal is reflected off the front surfaceof the device into the output.

FIG. 8 is a sectional view of a reflective based Fabry-Perot opticalswitch 200 in accordance with one embodiment of the present invention.Switch 200 includes saturable absorber material 205 between a frontmirror 206 and a rear mirror 207. A control beam 203 and an opticalinput signal 201 results in a switched optical output signal 202.

In another embodiment, the Fabry-Perot switch is fabricated on asubstrate via conventional thin film processing techniques. FIG. 9 is aperspective view of a Fabry-Perot switch 250 that is fabricated on asubstrate 215. Switch 250 includes a control beam waveguide 203, inputsignal waveguide 210 and output signal waveguide 211. As in switch 200of FIG. 8, saturable absorber 205 is placed between front mirror 206 andrear mirror 207. The waveguides may be composed of dielectric glass,polymer, or high bandgap semiconductor optical fiber (i.e., asemiconductor with a bandgap greater than that of the operationalwavelength making it transparent), or ridge, buried or photonic crystalbased waveguides composed of glass, polymers, or high bandgapsemiconductors.

FIG. 10 is a perspective view of a Fabry-Perot switch 260 in accordancewith another embodiment of the present invention. In switch 260, controlbeam signal 203 is delivered to saturable absorber 205 via an opticalfiber 220. Optical fiber 220 is perpendicular to the plane of substrate215.

FIG. 11 is a perspective view of a Fabry-Perot switch 270 in accordancewith another embodiment of the present invention. In switch 270, inputsignal 201 and output signal 202 propagate along optical fibers 222 and223, respectively. Optical fibers 222 and 223 are perpendicular to theplane of substrate 215.

The input and output data streams of the Fabry-Perot switch inaccordance with the present invention are either at some angle less than180° from one another (such as with switches 250, 260 and 270 of FIGS.9, 10 and 11, respectively) or counter-propagating (i.e., the input andoutput are in the same waveguide but propagate in different directions).In the counter-propagating embodiment, the input and output data signalsare separated via an optical isolating device such as a circulator. FIG.12 is a perspective view of a counter-propagating Fabry-Perot switch 280that includes a circulator 230 and an input/output waveguide 232.

FIG. 13 is a sectional view of a switch 290 in accordance with anotherembodiment of the present invention. Switch 290 includes an additionalset of mirrors 261 and 262 that are perpendicular to the propagationdirection of control beam 203. Mirrors 261 and 262 form anotherFabry-Perot cavity that acts to enhance the absorption of control beam203.

FIG. 14 is a sectional view of a switch 295 in accordance with anotherembodiment of the present invention. Switch 295 includes an opticalamplifier 292 that amplifiers control beam 203 and generates anamplified control beam 291.

The active region in the normally “off” configuration switches describedabove is designed so that the saturable absorber material is placedbetween two mirrors perpendicular to the propagation direction of theoptical data stream, or at an angle to the perpendicular to thepropagation direction of the data. In one embodiment, the rear mirrorhas a reflectivity of 1 so that none of the optical data stream istransmitted through it. The front mirror has a reflectivity between 0and 1. In general, the higher the front mirror reflectivity the lowerthe absorption required for the saturable absorber material when nocontrol light is applied. As previously shown in FIG. 7, the reflectedoptical power for any device having a front mirror reflectivity greaterthan 0 has a minimum point. For very high absorption coefficients of thesaturable absorber material, the reflection off the device approachesthat of the front mirror reflectivity and for very low absorptioncoefficients the reflected power approaches unity. The mirrorsthemselves can be made of dielectric thin films, thin metal layers orscattering elements in an integrated form acting as a mirror.

The Fabry-Perot switches operate with low field absorption coefficientof the saturable absorber material (i.e., low field absorptioncoefficient is the absorption coefficient of the saturable absorbermaterial with no control light incident upon it) at or near the minimumof the reflected power vs. the absorption coefficient curve when thereis no control beam incident upon the device. When the control beam isapplied to the device, the absorption coefficient is reduced and thepercentage of the data beam reflected off the device is increased. Inone embodiment, a Fabry-Perot switch having a front mirror reflectivityof 0.95 operates with an absorption coefficient of the saturableabsorber material of approximately 200 cm⁻¹ in the “off” state and anabsorption coefficient of 10 cm⁻¹ in the “on” state.

In one embodiment, the response of the Fabry-Perot switch is enhancedthrough the addition of several cavities in a row. Specifically thereflected output data beam from the first cavity becomes the input databeam for the next cavity. FIG. 15 is a graph illustrating the effect ofmultiple cavities on the device reflectivity.

Normally “On” Reflectivity Based Configuration

One embodiment of the present invention is a normally “on” Fabry-Perotbased saturable absorber all-optical switch that reflects the opticalsignal beam off the front mirror when there is no control beamsimultaneously incident upon the active region, and absorbs the opticaldata signal beam when there is an optical control beam incident upon theactive region. This embodiment is nearly identical to that of thenormally “off” switches described above except that the absorptioncoefficient of the saturable absorber material relative to the minimumreflected power is opposite that of the “normally off” device. Forexample the low field absorption (i.e., no control beam incident uponthe active region) of the saturable absorber material for a front mirrorreflectivity of 0.95 is 10000 cm⁻¹ corresponding to a reflectivity of0.9. When the control beam is incident upon the active region theabsorption coefficient is lowered and the reflectivity is subsequentlyreduced. If the absorption coefficient is reduced to approximately 200cm⁻¹ the reflectivity is reduced to approximately 0.009.

Normally “Off” Transmission Based Configuration

One embodiment of the present invention is a normally “off” transmissionbased Fabry-Perot switch that operates by altering the transparency ofthe cavity using a change in the absorption coefficient of the saturableabsorber material within the cavity. When a high intensity control beamis incident upon the saturable absorbing material the absorptioncoefficient is decreased. The decreased absorption coefficient has theeffect of making the Fabry-Perot device more transparent.

FIG. 16 is a sectional view of a normally “off” transmission Fabry-Perotswitch in accordance with one embodiment of the present invention.Switch 300 includes a saturable absorber material 320 placed between twoparallel mirrors 310 and 312 having a reflectivity greater than 0 andless than 1. Mirrors 310 and 312 can be comprised of dielectric thinfilms, Bragg type reflectors, or thin metal films. Switch 300 alsoincludes an input waveguide 301, output waveguide 302 and control beamwaveguide 308 that carry signals 305-307, respectively. Typically thebest results, or the greatest change in the transmission and aninsertion loss less than 1 dB for the “on” state, occur when both thefront and rear mirrors have the same reflectivity. In one embodiment,the thickness of saturable absorber material 320 (forming the cavity) isdesigned so that the device is resonant at the operational wavelength.The actual thickness is dependent upon the index of refraction of thematerial, but is in the range of 0.5-20 micrometers.

As with the previously described Fabry-Perot switches, switch 300 canalso include another set of control beam mirrors 314 and 316perpendicular to the propagation direction of the control beam, forminganother Fabry-Perot cavity that acts to enhance the absorption of thecontrol beam. In addition, switch 300 can be fabricated on a substrateusing conventional thin film growth techniques. The mirrors may be growneither perpendicular to the substrate or parallel to it. In the formerembodiment, both the input and output waveguides are in the plane of thesubstrate, and the control beam may also be in the plane of thesubstrate and perpendicular to the input/output waveguides, or out ofthe plane of the substrate, delivered to the active region of the switchvia an optical fiber. In the latter embodiment, where the mirrors aregrown parallel to the plane of the substrate, the input signal isdelivered perpendicular to the plane of the substrate via an opticalfiber and the output signal is on the opposite side of the substrate,which is transparent to the optical data signal. The control beam inthis embodiment is delivered via optical waveguide in the plane.

Fabry-Perot Cross-Connect Switch

FIG. 17 is a sectional view of a Fabry-Perot switch in accordance withanother embodiment of the present invention. Switch 400 is anall-optical cross connect switch (i.e., a 1×2 switch where the inputoptical signal is switched to one of two outputs) that is fabricatedusing a Fabry-Perot microcavity, formed with a front mirror 402, a rearmirror 403 and a saturable absorber material 400. In this architecture aFabry-Perot microcavity is placed at the intersection of an inputoptical waveguide 405 and two output optical waveguides 407 and 408, allof which are in the same plane. Output optical waveguide 408 is placedopposite to input optical waveguide 405. Output optical waveguide 407 isplaced at an angle from input optical waveguide 405. The angle betweeninput waveguide 405 and output waveguide 407 is substantially equal totwice the angle of input waveguide 405 with the perpendicular of frontmirror 402 of the microcavity.

When saturable absorber material 401 is not exposed to a high intensityoptical control beam in control beam waveguide 410, it is highlyabsorbing and therefore the Fabry-Perot microcavity is highlyreflecting. In this case, the optical signal input is reflected off thedevice and into output 407. However, when saturable absorber material401 is illuminated with the high intensity optical control beam, theabsorption is increased causing the transparency of the device to beincreased as well (the reflectivity is decreased). In this situation theoptical input signal is no longer reflected into output 407 but insteadtransmitted through the device into output 408. As in the previousdescribed devices, switch 400 can be formed on a substrate, and theinput, output and control beam waveguides can be formed with opticalfiber.

Several embodiments of the present invention are specificallyillustrated and/or described herein. However, it will be appreciatedthat modifications and variations of the present invention are coveredby the above teachings and within the purview of the appended claimswithout departing from the spirit and intended scope of the invention.

For example, although a relatively simple on-off optical switch has beendescribed, the switch can function as a transistor, and can form thebasis for more complex optical logic and computing circuitry. In turn,the logic circuitry can be used to accomplish higher-levelcommunications protocols including optical time domain multiplexing andoptical packet switching that are currently unattainable with knowntechnologies. In addition, the all-optical switch in accordance with thepresent invention could form the basis for optical Read Only Memories(“ROM”s) and Programmable Read Only Memories (“PROM”s) and combinatorialand synchronous circuits including adding circuitry, flip-flops,counters, and registers. Therefore, a computer system can include, forexample, a processor formed from the optical switches, and memory formedfrom the optical switches.

What is claimed is:
 1. A Fabry-Perot optical switch comprising: asaturable absorber comprising quantum dots; a front mirror coupled tosaid saturable absorber; a rear mirror coupled to said saturableabsorber; an input waveguide coupled to said saturable absorber; anoutput waveguide coupled to said saturable absorber; and a control beamwaveguide coupled to said saturable absorber.
 2. The Fabry-Perot opticalswitch of claim 1, wherein said saturable absorber comprises a pluralityof electrons having a first and a second state, and wherein saidelectrons are in said first state when substantially no light is inputto said control beam waveguide, and a portion of said electrons are insaid second state when light is input to said control beam waveguide. 3.The Fabry-Perot optical switch of claim 2, wherein said first state is alower energy state, and said second state is an upper energy state. 4.The Fabry-Perot optical switch of claim 1, wherein said quantum dotscomprise Lead Sulfide.
 5. The Fabry-Perot optical switch of claim 1,further comprising a substrate coupled to said input waveguide, saidoutput waveguide, and said control beam waveguide.
 6. The Fabry-Perotoptical switch of claim 5, wherein said control beam waveguide and saidsubstrate are on a first plane.
 7. The Fabry-Perot optical switch ofclaim 5, wherein said control beam waveguide is on a first plane andsaid substrate is on a second plane, wherein said first plane and saidsecond plane are different planes.
 8. The Fabry-Perot optical switch ofclaim 1, wherein said saturable absorber comprises cladding coupled tosaid quantum dots.
 9. The Fabry-Perot optical switch of claim 8 whereinsaid quantum dots are manufactured using a colloidal growth process. 10.The Fabry-Perot optical switch of claim 1, further comprising an opticalamplifier coupled to said control beam waveguide.
 11. The Fabry-Perotoptical switch of claim 1, further comprising an optical circulatorcoupled to said input waveguide and said output waveguide.
 12. TheFabry-Perot optical switch of claim 1, wherein said front mirror has areflectivity between 0 and 1, and said rear mirror has a reflectivitysubstantially equal to
 1. 13. The Fabry-Perot optical switch of claim 1,wherein said front mirror has a reflectivity greater than 0 and lessthan 1, and said rear mirror has a reflectivity greater than 0 and lessthan
 1. 14. The Fabry-Perot optical switch of claim 1, furthercomprising a pair of control beam mirrors coupled to said saturableabsorber.
 15. The Fabry-Perot optical switch of claim 1, furthercomprising a second output waveguide coupled to said saturable absorber.